The present invention pertains to apparatus used in seismic exploration and more particularly to apparatus used in the collection of seismic acoustic pulses at a point remote from the generation of seismic acoustic pulses.
In most seismic exploration, an acoustic pulse generator is used to transmit acoustic pulses into the earth's subsurface formations. Whenever an acoustic pulse is incident upon a less dense/more dense interface, a portion of this pulse is reflected back to the surface while the remainder continues on its path prior to incidence or refracted along the formation. The refracted portion may travel along the formation and emerge as an acoustic pulse traveling in the same direction as the reflected wave. The reflected and refracted portions of the transmitted acoustic pulses are detected by geophones or the like which transduce the acoustic pulses to electrical signals. The electrical signals may be recorded or fed to a display such as a cathode ray tube. In general the electrical signals are recorded for further processing to remove undesireable portions. In general, the recordings of seismic data have been operator controlled. The operator rewinds the tape or record to a predetermined starting point after he or she energizes the recording unit. For unidirectional recorders, a track is recorded, the tape is rewound and an additional track is recorded or a new reel of tape is installed. The operator monitors the track so that transition to additional tracks may be performed when the initial track has been completed. In some instances, recorders may record in only one direction, such as forward, but some recorders will record in reverse. In such cases, the operator must switch the recording operation not only to change tracks but to change recording direction when the end of tape (EOT) is reached.
Prior art attempts at remote operation have, in general, been limited in deployment time due to high power requirements and lack of intelligence capability.
In present seismic exploration, rising costs have created a need for the collection of a greater amount of data for larger areas than previously obtainable. In general, seismic exploration comprises a seismic pulse generator and seismic pulse detectors. The seismic pulse generator is used to transmit acoustic pulses into the earth's formation which are reflected and refracted by subsurface formation density changes called interfaces and travel towards the surface. Seismic pulse detectors receive these reflected and refracted acoustic pulses and produce an electric signal therefrom. Often seismic detectors are located several miles from the acoustic pulse generators. Apparatus and methods for collecting data at locations remote from the genertion of acoustic seismic energy is taught in copending U.S. application Ser. No. 163,757 entitled On-Bottom Seismometer Electronic System now U.S. Pat. No. 4,422,164. However, recording data at remote, unmanned locations presents several problems to which present technology has no readily available answers.
The first problem encountered in a remotely operated, unmanned recording unit is that of power consumption. Units capable of recording sophisticated seismic data require a substantial power supply to operate computer storage units, to drive the recording unit and to provide power for seismic signal detectors. A remote unit must be ready to record from the time it is placed in position and personnel leave. Since the lead time between deployment of the remote unit and the initiation of seismic pulse generation may be several hours, the unit will have consumed a significant amount of power prior to actual recordation.
A second problem encountered is the determination of the location on the tape where the seismic record begins. It is possible for tape cartridges to be loaded in the recorder and not be fixed at a specific starting position, namely, the beginning of the tape. As a result, much recording capability can be lost. This situation is particularly true where a plurality of remote units are being loaded in preparation for deployment.
An additional problem encountered with recording seismic data is a property of recording units capable of recording seismic data in digital format. In general, the recorder is started and records data until it reaches the end of the tape. Without personnel to change the tape or to rewind the tape, the recording is terminated.
A further problem in recording occurs if a feature wherein the tape is automatically rewound at the remote location is incorporated into the system. This problem is a determination of whether the tape is being run in the forward or reverse direction. The lack of direction sensing capability limits the possible functions of the recording unit. If, for example, a microprocessor unit is used to control the tape recorder and the tape has several tracks, the system is incapable of determining whether the tape recorder is recording or rewinding.